The present invention relates to a data recording medium. The data recording medium has a radiation-sensitive recording layer. A laser beam is focused on the radiation-sensitive recording layer to form a pit therein, thereby recording optical data. The optical data recorded in this manner can be read out.
FIG. 1 is a sectional view showing an example of a conventional data recording medium. A data recording medium 1 of this example has two disc-shaped transparent substrates 3 of synthetic resin, on each of which is formed a recording layer 2 of a light-reflecting material. These transparent substrates 3 are opposed to each other such that the recording layers 2 face each other. The transparent substrates 3 are bonded to each other through inner and outer spacers 5a and 5b and an adhesive so as to define a hollow space 4 therebetween.
However, the data recording medium of this type has drawbacks to be described below. For example, since the writing and reading of data in and out of the recording layers 2 is performed through the intermediacy of the transparent substrates 3, the transparent substrates 3 cannot be made very thick. At the same time, these two transparent substrates 3 are bonded to each other through the inner and outer spacers 5a and 5b only at the inner and outer peripheries thereof. Therefore, each transparent substrate is weak in mechanical strength and is thus subject to bending by external forces. In addition to this, the transparent substrate 3 has a relatively small elasticity. When the data recording medium is left to stand obliquely for a long period of time, the medium becomes distorted due to its own weight. Such a distortion persists after the data recording medium is removed from this condition. When the part of the data recording medium separated from the inner and outer spacers 5a and 5b (nonbonded part) is clamped and pressed by the fingers from both sides, the hollow space 4 is eliminated. The opposing recording layers 2 then contact each other and are subject to damage. Furthermore, the nonbonded part of the data recording medium is also subject to undulations and deflections. When the data recording medium is stored horizontally, the upper transparent substrate may become indented due to its own weight. Since the inner and outer spacers 5a and 5b are incorporated, the manufacturing cost is increased. When the inner and outer spacers 5a and 5b are bonded to the transparent substrates 3, care must be taken not to allow overflow of the adhesive into the hollow space 4. This results in a laborious bonding step. As may be easily understood, since the transparent substrates 3 are bonded only at the contact parts corresponding to the narrow inner and outer spacers 5a and 5b, the contact areas are small and the adhesion strength between the transparent substrate 3 is weak. If a gas is filled in the hollow space 4, a pressure difference is established between the outer atmosphere and the hollow space 4 between the transparent substrates 3 when the pressure or temperature in the outer atmosphere changes. This may result in deformation of the transparent substrates. When a plastic material is used for the transparent substrates 3, the transparent substrates 3 tend to allow water to permeate easily, and water may permeate from the outer atmosphere to the hollow space 4 and vice versa. Before the water content which has permeated through the transparent substrate 3 reaches the hollow space 4, it also permeates through the recording layer 2 and degrades the recording layer 2. When the data recording medium is left to stand in an atmosphere at high temperature, the humidity in the hollow space 4 is increased. If the data recording medium is rapidly cooled from the outside under this state, condensate may deposit on the surface of the recording layers 2. The plastic material of the transparent substrates 3 also allows air to permeate easily. Therefore, the hollow space 4 cannot be evacuated or filled with an inert gas in order to achieve a long service life of the recording layers 2, hence, the service life of the data recording medium cannot be prolonged.
A data recording medium as shown in FIG. 2 or 3 is proposed in order to solve these problems. FIG. 2 is a sectional view of an example of a data recording medium. In the data recording medium of this example, a recording layer 11 of Te is formed by vacuum deposition on one surface each of a pair of transparent substrates 10 of an acrylic resin, for example. These recording layers 11 are opposed to each other and then the transparent substrates 10 are bonded through an adhesive layer 12 of an epoxy-type adhesive. FIG. 3 is a sectional view showing another example of a data recording medium. In the data recording medium of this example, after an indented layer 13 with tracking guides etc. is formed on the surface of the transparent substrate 10, the recording layer 11 is formed thereover. The transparent substrates 10 thus obtained are bonded to each other through the adhesive layer 12 of the epoxy-type adhesion so that the recording layers 11 face each other.
However, in the data recording medium of this type, the transparent substrates 10 are bonded to each other by directly applying the adhesive to the recording layers 11. Therefore, during the step of uniformly applying the adhesive, part of the recording layer 11 may peel off, thus adversely affecting the recording layers and providing obstructions to mass production of the data recording media.
As may be seen from FIGS. 2 and 3, since the recording layers 11 and the adhesive layer 12 are in direct contact with each other, the recording layers 11 may be adversely affected by certain components of the adhesive layer 10.